Flag bit generation

1、 Overflow sign OF (Overflow Flag)
If the result of arithmetic operation overflows , be OF＝1; otherwise OF＝0
2、 sign indicator SF (Sign Flag)
The highest bit of the operation result is 1, be SF = 1; otherwise SF = 0
3、 Zero mark ZF (Zero Flag)
If the result is 0, be ZF = 1; otherwise ZF = 0
4、 Carry mark CF (Carry Flag)
When the most significant bit of the operation result has a carry （ Add ） Or borrow a seat （ Subtraction ） when , The carry flag is set to 1, namely CF = 1; otherwise CF= 0.